Answer: A small band of rebels from Yavin 4. Eventually laid down there lives to the power of the Death Star "Rogue One". The plans made it past Scarif but were eventually captured by a imperial star destroyer "A New Hope". They escaped and then Star Wars happened.
The plans contain the Death Star's entire design history, including the flaw introduced into its reactor system. Stolen by Jyn Erso, they are beamed to the Profundity, copied and hidden in the memory systems of a humble astromech droid.
